Intercropping of Cyphomandra betacea with Different Ploidies of Solanum Sect. Solanum (Solanaceae) Wild Vegetables Increase Their Selenium Uptakes

Selenium (Se) deficiency causes various diseases in humans. Se can be obtained from fruits and vegetables. In this study, the fruit tree Cyphomandra betacea was intercropped with three Solanum sect. Solanum (Solanaceae) wild vegetables [diploid (S. photeinocarpum), tetraploid (colchicine-induced S. photeinocarpum), and hexaploid (S. nigrum)], respectively, and Se uptakes of these plants were determined by a pot experiment. Intercropping decreased the biomass, photosynthetic pigment content, and superoxide dismutase activity of C. betacea, but increased the peroxidase (POD) activity, catalase (CAT) activity, and soluble protein content of C. betacea. These indicators’ values of sect. Solanum increased after intercropping. The contents of Se increased in C. betacea and sect. Solanum after intercropping. Intercropped with diploid, tetraploid, and hexaploid increased the shoot Se contents in C. betacea by 13.73%, 17.49%, and 26.50%, respectively, relative to that of C. betacea monoculture. Intercropped with C. betacea increased the shoot Se contents in diploid, tetraploid, and hexaploid by 35.22%, 68.86%, and 74.46%, respectively, compared with their respective monoculture. The biomass and Se content of intercropped sect. Solanum showed linear relationships with the biomass and Se content of their monocultures. The biomass and Se content of intercropped C. betacea also exhibited linear relationships with that of sect. Solanum monocultures. Correlation and grey relational analyses revealed that the CAT activity, POD activity, and soluble protein content were the top three indicators closely associated with the C. betacea shoot Se content. The POD activity, soluble protein content, and translocation factor were the top three indicators closely associated with sect. Solanum shoot Se content. Therefore, intercropping can promote the Se uptake in C. betacea and sect. Solanum wild vegetables.


Introduction
Selenium (Se) is an essential trace element in humans. Humans obtain less than 40 µg d −1 of Se from the diet, and Se poisoning occurs after intake of concentration above 400 µg d −1 [1,2]. Se deficiency is associated with various human diseases such as heart disease, cancer, and reproductive disorders. High intake of Se in humans leads to loss of hairs and nails, and damage to the nervous and digestive systems [2]. The human body cannot synthesize Se; so, it is obtained through diet. Plant Se is the primary source of Se in humans [3,4]. Application of Se fertilization is a very common method for improving plant Se contents, but this approach is expensive with high environmental pollution risk [5]. Therefore, exploring effective and eco-friendly methods to improve Se accumulation in crops is imperative.
Intercropping can improve the rate of utilization of environmental resources by crops to some extent. Moreover, intercropping also can modulate the soil environment conditions,

Biomass of Plants
The root and shoot biomasses of C. betacea were lower than that of diploid, tetraploid, and hexaploid. The biomass order in different ploidies was diploid < tetraploid < hexaploid ( Figure 1A,B). Compared with C. betacea monoculture, intercropped with diploid, tetraploid, and hexaploid decreased the root biomass of C. betacea by 6.07%, 15.63%, and 19.23%, respectively, and decreased the shoot biomass of C. betacea by 9.30%, 20.15%, and 23.13%, respectively. The root and shoot biomasses of different ploidies in intercropping were higher than that of their respective monoculture. In addition, the root biomasses of intercropped diploid, tetraploid, and hexaploid were linearly positively correlated with the root biomasses of their respective monoculture (r = 0.704, n = 9, p = 0.034, Figure 2A). The root biomasses of intercropped C. betacea were negatively correlated with the root biomasses of monoculture diploid, tetraploid, and hexaploid (r = −0.896, n = 9, p = 0.001, Figure 2B). The shoot biomasses of intercropped diploid, tetraploid, and hexaploid were positively related to the shoot biomasses of their corresponding monoculture (r = 0.922, n = 9, p = 0.000, Figure 2C). The shoot biomasses of intercropped C. betacea were linearly negatively correlated with the shoot biomasses of monoculture diploid, tetraploid, and hexaploid (r = −0.796, n = 9, p = 0.010, Figure 2D).

Photosynthetic Pigment Contents in Plants
Intercropped with diploid, tetraploid, and hexaploid showed lower contents of chlorophyll a, chlorophyll b, and carotenoid in C. betacea compared with the levels in C. betacea monoculture (Table 1). Notably, the chlorophyll a/b of C. betacea in intercropping was not significantly different with the level in monoculture. Intercropping showed no significant differences in the chlorophyll a and carotenoid contents in diploid relative to that of the diploid monoculture. The content of chlorophyll b in diploid was higher in intercropping, and the chlorophyll a/b of diploid was lower, compared with that of diploid monoculture. Tetraploid in intercropping showed higher contents of chlorophyll a, chlorophyll b, and carotenoid relative to that of tetraploid monoculture, whereas there was no significant difference in chlorophyll a/b between tetraploid intercropping and tetraploid monoculture. Hexaploid in intercropping exhibited higher levels of chlorophyll a and carotenoid compared with that in hexaploid monoculture. On the contrary, chlorophyll b content and chlorophyll a/b of hexaploid were not significantly different between monoculture and intercropping. intercropped diploid, tetraploid, and hexaploid were linearly positively correlated with the root biomasses of their respective monoculture (r = 0.704, n = 9, p = 0.034, Figure 2A). The root biomasses of intercropped C. betacea were negatively correlated with the root biomasses of monoculture diploid, tetraploid, and hexaploid (r = −0.896, n = 9, p = 0.001, Figure 2B). The shoot biomasses of intercropped diploid, tetraploid, and hexaploid were positively related to the shoot biomasses of their corresponding monoculture (r = 0.922, n = 9, p = 0.000, Figure 2C). The shoot biomasses of intercropped C. betacea were linearly negatively correlated with the shoot biomasses of monoculture diploid, tetraploid, and hexaploid (r = −0.796, n = 9, p = 0.010, Figure 2D).

Photosynthetic Pigment Contents in Plants
Intercropped with diploid, tetraploid, and hexaploid showed lower contents of chlorophyll a, chlorophyll b, and carotenoid in C. betacea compared with the levels in C. betacea monoculture (Table 1). Notably, the chlorophyll a/b of C. betacea in intercropping was not significantly different with the level in monoculture. Intercropping showed no significant differences in the chlorophyll a and carotenoid contents in diploid relative to that of the diploid monoculture. The content of chlorophyll b in diploid was higher in intercropping, and the chlorophyll a/b of diploid was lower, compared with that of diploid monoculture. Tetraploid in intercropping showed higher contents of chlorophyll a, chlorophyll b, and carotenoid relative to that of tetraploid monoculture, whereas there was no significant difference in chlorophyll a/b between tetraploid intercropping and tetraploid monoculture. Hexaploid in intercropping exhibited higher levels of chlorophyll a and carotenoid compared with that in hexaploid monoculture. On the contrary, chlorophyll b content and chlorophyll a/b of hexaploid were not significantly different between monoculture and intercropping.

Antioxidant Enzyme Activities and Soluble Protein Contents in Plants
Intercropped with diploid and tetraploid did not exhibit significant differences in SOD activity of C. betacea compared with C. betacea monoculture ( Table 2). Intercropped with hexaploid showed lower SOD activity of C. betacea than that of C. betacea monoculture. Intercropped with diploid, tetraploid, and hexaploid showed higher POD activity, CAT activity, and soluble protein content of C. betacea compared with the C. betacea monoculture. Diploid intercropped with C. betacea showed higher SOD activity, POD activity, CAT activity, and soluble protein content of diploid compared with diploid monoculture. Tetraploid intercropped with C. betacea did not exhibit significant differences in SOD activity of tetraploid compared with tetraploid monoculture. However, tetraploid intercropped with C. betacea had higher POD activity, CAT activity, and soluble protein content of tetraploid relative to tetraploid monoculture. Hexaploid intercropped with C. betacea did not exhibit significant differences in SOD activity and CAT activity of hexaploid relative to hexaploid monoculture. Compared with the hexaploid monoculture, intercropped with C. betacea had higher POD activity and soluble protein content of hexaploid.

Se Content and Translocation Factor of Plants
C. betacea intercropped with diploid, tetraploid, and hexaploid exhibited higher root and shoot Se contents compared with that of their respective monoculture ( Figure 3A,B). Intercropped with diploid, tetraploid, and hexaploid increased the Se content in shoots of C. betacea by 13.73%, 17.49%, and 26.50%, respectively, compared with C. betacea monoculture. Intercropped with C. betacea increased the shoot Se contents in diploid, tetraploid, and hexaploid by 35.22%, 68.86%, and 74.46%, respectively, compared with their respective monoculture. In addition, the root Se contents in diploid, tetraploid, and hexaploid in intercropping were positively correlated with their root Se contents in monoculture (r = 0.760, n = 9, p = 0.018, Figure 4A). The root Se contents in C. betacea in intercropping were positively correlated with the root Se contents in diploid, tetraploid, and hexaploid in monoculture (r = 0.739, n = 9, p = 0.023, Figure 4B). The shoot Se contents in diploid, tetraploid, and hexaploid in intercropping were linearly positively correlated with their shoot Se contents in monoculture (r = 0.927, n = 9, p = 0.000, Figure 4C). The shoot Se contents in C. betacea in intercropping were linearly positively correlated with the shoot Se contents in diploid, tetraploid, and hexaploid in monoculture (r = 0.958, n = 9, p = 0.000, Figure 4D).
intercropping were positively correlated with their root Se contents in monoculture (r = 0.760, n = 9, p = 0.018, Figure 4A). The root Se contents in C. betacea in intercropping were positively correlated with the root Se contents in diploid, tetraploid, and hexaploid in monoculture (r = 0.739, n = 9, p = 0.023, Figure 4B). The shoot Se contents in diploid, tetraploid, and hexaploid in intercropping were linearly positively correlated with their shoot Se contents in monoculture (r = 0.927, n = 9, p = 0.000, Figure 4C). The shoot Se contents in C. betacea in intercropping were linearly positively correlated with the shoot Se contents in diploid, tetraploid, and hexaploid in monoculture (r = 0.958, n = 9, p = 0.000, Figure 4D).   The findings showed that intercropped with diploid had higher TF of C. betacea relative to C. betacea monoculture ( Figure 3C). Intercropped with tetraploid showed no significant difference in TF of C. betacea, whereas intercropped with hexaploid had lower TF of C. betacea compared with C. betacea monoculture. Diploid, tetraploid, and hexaploid in intercropping all exhibited higher TFs compared with their TFs in monoculture.  The findings showed that intercropped with diploid had higher TF of C. betacea relative to C. betacea monoculture ( Figure 3C). Intercropped with tetraploid showed no significant difference in TF of C. betacea, whereas intercropped with hexaploid had lower TF of C. betacea compared with C. betacea monoculture. Diploid, tetraploid, and hexaploid in intercropping all exhibited higher TFs compared with their TFs in monoculture.

Relationship of Different Parameters in Plants
The root biomass and shoot biomass of C. betacea were highly significantly (p < 0.01) positively correlated with the chlorophyll a content, chlorophyll b content, carotenoid content, and SOD activity ( Table 3). The root biomass and shoot biomass were highly significantly (p < 0.01) negatively correlated with the POD activity, CAT activity, and soluble protein content. The root Se content and shoot Se content were highly significantly (p < 0.01) positively correlated with the POD activity, CAT activity, and soluble protein content, and were highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) negatively correlated with the root biomass, shoot biomass, chlorophyll a content, chlorophyll b content, carotenoid content, and SOD activity. The root Se content was highly significantly (p < 0.01) positively correlated with the shoot Se content.
The root biomass of diploid was highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) positively correlated with the POD activity, CAT activity, soluble protein content, and TF ( Table 4). The shoot biomass was highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) positively correlated with the CAT activity, soluble protein content, and TF. The root Se content was highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) positively correlated with the root biomass, carotenoid content, POD activity, and soluble protein content. The shoot Se content was highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) positively correlated with the root biomass, shoot biomass, chlorophyll b content, POD activity, CAT activity, soluble protein content, and TF, and was significantly (0.01 ≤ p < 0.05) negatively correlated with the content of chlorophyll a/b. The root Se content was highly significantly (p < 0.01) positively correlated with the shoot Se content.
The root biomass of tetraploid was significantly (0.01 ≤ p < 0.05) positively correlated with the chlorophyll a content, chlorophyll b content, carotenoid content, CAT activity, and soluble protein content ( Table 5). The shoot biomass was highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) positively correlated with the chlorophyll b content, POD activity, soluble protein content, and TF. The root Se content and shoot Se content were highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) positively correlated with the root biomass, shoot biomass, chlorophyll a content, chlorophyll b content, carotenoid content, POD activity, CAT activity, soluble protein content, and TF. The root Se content was highly significantly (p < 0.01) positively correlated with the shoot Se content.
The root biomass of hexaploid was highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) positively correlated with the chlorophyll a content, SOD activity, POD activity, soluble protein content, and TF ( Table 6). The shoot biomass was highly significantly (p < 0.01) positively correlated with the POD activity, soluble protein content, and TF. The root Se content was highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) positively correlated with the root biomass, shoot biomass, carotenoid content, POD activity, soluble protein content, and TF. The shoot Se content was highly significantly (p < 0.01) or significantly (0.01 ≤ p < 0.05) positively correlated with the root biomass, shoot biomass, chlorophyll a content, carotenoid content, POD activity, soluble protein content, and TF. The root Se content was highly significantly (p < 0.01) positively correlated with the shoot Se content.

Grey Relational Analysis
The grey relationships of different indicators with the shoot Se content were evaluated and the results showed that all indicators were correlated with the shoot Se content ( Figure 5). The top four indicators of C. betacea with the highest grey correlation coefficients were CAT activity, POD activity, soluble protein content, and root Se content ( Figure 5A). The top four indicators with the highest grey correlation coefficients in diploid were POD activity, soluble protein content, TF, and root biomass ( Figure 5B). The top four indicators for tetraploid were soluble protein content, POD activity, root Se content, and TF ( Figure 5C), and the top four indicators for hexaploid were soluble protein content, shoot biomass, TF, and POD activity ( Figure 5D).

Grey Relational Analysis
The grey relationships of different indicators with the shoot Se content were evaluated and the results showed that all indicators were correlated with the shoot Se content ( Figure 5). The top four indicators of C. betacea with the highest grey correlation coefficients were CAT activity, POD activity, soluble protein content, and root Se content (Figure 5A). The top four indicators with the highest grey correlation coefficients in diploid were POD activity, soluble protein content, TF, and root biomass ( Figure 5B). The top four indicators for tetraploid were soluble protein content, POD activity, root Se content, and TF ( Figure 5C), and the top four indicators for hexaploid were soluble protein content, shoot biomass, TF, and POD activity ( Figure 5D).

Discussion
Intercropping can change the soil organic acid content, soil pH value, soil nutrient availability, and soil enzyme activity, ultimately affecting the nutrient uptake and growth of plants [22]. In addition, intercropping results in competition between plants; this may inhibit the growth of the plants [23]. Different species of intercropped eggplants exhibited an increase in the biomasses of their seedlings under Se-rich soil conditions [12], whereas intercropping of white radish with green radish resulted in an increase in the biomass of white radish and a decrease in the biomass of green radish [13]. In the current study, C. betacea intercropped with sect. Solanum showed a decrease in the biomass of C. betacea and an increase in the biomass of sect. Solanum. This result is consistent with the findings from previous studies [13,14], but different from results from other studies [9,10,12]. This finding indicates competition for growth resources between C. betacea and sect. Solanum. The biomass of sect. Solanum was higher than that of C. betacea in the present study. Sect. Solanum are herbs, and their growth rates are higher than that of the seedlings of C. betacea, a woody plant. Therefore, sect. Solanum had a faster growth than C. betacea, inhibiting the growth of C. betacea. The biomass order of sect. Solanum was diploid < tetraploid < hexaploid and intercropping of C. betacea also showed that the biomass order was diploid < tetraploid < hexaploid. This finding implies that sect. Solanum competed and inhibited the growth of C. betacea. The biomass of intercropped sect. Solanum showed a positive linear relationship with their biomass of monoculture. The biomass of intercropped C. betacea exhibited a linear negative relationship with that of the sect. Solanum monoculture. This result further indicates the growth competition between C. betacea and sect. Solanum. Further studies should be conducted to explore the competition mechanism between the two species.
The content of photosynthetic pigments in plants reflects the photosynthetic capacity [24]. Eggplant intercropped with the Cd-hyperaccumulator S. nigrum exhibited a decrease in the photosynthetic pigment content in eggplant planted in Cd-contaminated soil. Tomato intercropped with S. nigrum under the same conditions showed an increase in the photosynthetic pigment content in tomato [25]. Grape intercropped with S. nigrum planted in Cd-contaminated soil showed increased chlorophyll a and total chlorophyll contents in grape [26]. Maize intercropped with peanut and maize intercropped with soybean showed increased chlorophyll content in maize [7,27]. In the current study, intercropping of C. betacea with sect. Solanum exhibited a decrease in the photosynthetic pigment content in C. betacea and an increase in the photosynthetic pigment content in sect. Solanum. These results indicate that the competition between C. betacea and sect. Solanum inhibited synthesis of photosynthetic pigments in C. betacea and promoted the synthesis of photosynthetic pigments in sect. Solanum, which may be related to the competition for light, nutrients, and water resources.
High concentrations of Se in plants promote the production of reactive oxygen species (ROS) and induce oxidative stress in plants [28]. Plant defense mechanisms to alleviate Se stress include activating different antioxidant enzymes such as SOD, POD, and CAT [28,29]. Soluble protein is an essential osmotic regulator of plant cells and is implicated in plant stress resistance [30]. Eggplant intercropped with S. nigrum in Cd-contaminated soil showed increased antioxidant enzyme activity and soluble protein content of eggplant [31]. Lettuce intercropped with the Cd-hyperaccumulator G. parviflora exhibited increased antioxidant enzyme activity and soluble protein content of lettuce [32]. In the present study, intercropping of C. betacea with diploid and tetraploid had no significant effects on the SOD activity of C. betacea, whereas intercropping of C. betacea with hexaploid decreased SOD activity of C. betacea. Intercropping C. betacea with diploid, tetraploid, and hexaploid increased the POD activity, CAT activity, and soluble protein content of C. betacea. Intercropping sect. Solanum with C. betacea increased the antioxidant enzyme activity and soluble protein content of sect. Solanum. These results indicate that intercropping of C. betacea with sect. Solanum can improve the resistance of the two plants to Se stress, consistent with findings from previous studies [12][13][14]. Moreover, C. betacea intercropped with sect. Solanum may promote the communication of matters between two plants, resulting in the transfer of the active compounds to each other to mutually improve their resistance to Se stress [33], which needs to be further studied.
The uptake of Se by plants is modulated by various factors such as soil pH value, soil redox conditions, and soil selenium state [4]. Se has a strong metalloid property and exists in inorganic forms with different oxidation states. The different soil oxidation states modulate the various valence states of Se [34,35]. The pathway for absorption of selenite by plants is similar to the absorption pathway for phosphate. Absorption of selenite by plants mainly occurs through the roots [35,36]. On the other hand, there is an antagonism between sulphate and both selenate and selenite forms [37]. Intercropping improves the absorption of nutrients such as nitrogen, phosphorus, and potassium in maize and legumes [7,8,38,39]. Intercropping of heavy metal hyperaccumulator with nonheavy metal hyperaccumulator increases heavy metal uptake in hyperaccumulator but decreases heavy metal uptake in non-hyperaccumulator [9,40]. Intercropping of different genotypes of tomato, eggplant, and radish with themselves exhibits a decrease or increase in Se uptake [12][13][14]. In the current study, intercropping of C. betacea with sect. Solanum increased the Se uptake in the two plants, consistent with findings on the other plants planted in Cd-contaminated soil [10]. These findings are consistent with results on the uptake levels of Se in tomato, eggplant, and radish reported in previous studies [12][13][14]. The Se contents in intercropped sect. Solanum exhibited a positive linear relationship with their Se contents in monoculture. The Se content of intercropped C. betacea showed a linear negative relationship with the Se contents in sect. Solanum in monoculture. These results indicate that intercropping promoted the Se uptakes in C. betacea and sect. Solanum, further confirming the communication of matters between C. betacea and sect. Solanum, and several active compounds may be exchanged between C. betacea and sect. Solanum to improve the Se accumulation capacity of the two plants. Intercropping of C. betacea with diploid increased the TF of C. betacea, intercropping with tetraploid had no significant effect on the TF of C. betacea, whereas intercropping with hexaploid decreased the TF of C. betacea. However, intercropping of sect. Solanum with C. betacea increased the TFs of sect. Solanum. These results indicate that C. betacea intercropped with sect. Solanum had significant Se effects on sect. Solanum than C. betacea. This finding explains the higher percentage of shoot Se content in sect. Solanum than in C. betacea. Correlation and grey relational analyses revealed that CAT activity, POD activity, and soluble protein content were the top three indicators significantly associated with C. betacea shoot Se content. In contrast, POD activity, soluble protein content, and TF were the top three indicators markedly associated with sect. Solanum shoot Se content. These results further imply that intercropping promoted the Se absorption by increasing the resistance of the two plants to Se, and their action mechanisms should be explored further.

Materials
The C. betacea seeds were collected from a five-year-old fruiting tree growing at the Chengdu Campus of Sichuan Agricultural University (30 • 42 N, 103 • 51 E) in September 2021. The seeds were air-dried. The seeds of S. photeinocarpum (diploid) and S. nigrum (hexaploid) were obtained from the farmlands around the Chengdu Campus in June 2021. The shoot tips of the diploid seedlings at the stage of unfolded cotyledons were treated with colchicine (0.3%) for 48 h, and the tetraploid was induced. Tetraploid seeds were obtained from the diploid treated with colchicine in September 2021. Diploid, tetraploid, and hexaploid seeds were air-dried. The seeds of four plants were planted in seedling trays filled with wet perlite in February 2022. Hoagland solutions were used to irrigate the seedlings every three days after emergence.
The soil used in this experiment was collected from the farmland around the Chengdu Campus. The soil type was fluvo-aquic soil, and the basic physicochemical properties of soil samples are presented in Table 7 [41].

Experimental Design
The experiment was conducted in a greenhouse at the Chengdu Campus. The soil was treated in March 2022 according to   [42]. Soil with 3.0 kg was placed in a plastic pot (21 cm diameter and 20 cm depth), and Na 2 SeO 3 (analytical grade) was added to the soil to obtain 5 mg kg −1 Se concentration [43]. Then, the soil was watered to maintain the soil moisture content at 80% of the field capacity for one month. In April 2022, uniform plant seedlings were transplanted in the pot. In each pot, four seedlings were planted with even distribution in all four directions. The experiment comprised seven treatments: C. betacea monoculture, diploid monoculture, tetraploid monoculture, hexaploid monoculture, C. betacea intercropped with diploid, C. betacea intercropped with tetraploid, and C. betacea intercropped with hexaploid. Four seedlings of one plant species were planted in each pot for the monoculture treatments. Two seedlings of C. betacea and two seedlings of each diploid, tetraploid, or hexaploid were planted together in each pot for the intercropping treatments. Each treatment was conducted in triplicate (three pots), and a randomized block design was used in the study. The plants were irrigated using tap water.

Determination of Indicators
To determine whether the intercropping could improve the resistances of these plants to Se, the third mature leaf of each plant from the top was collected to determine various parameters, including the contents of photosynthetic pigments (chlorophyll a, chlorophyll b, and carotenoid), activities of antioxidant enzymes [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)], and soluble protein content following the methods reported by Lin et al. (2023) [44] and Hao et al. (2004) [45] two months after plants transplanting. Subsequently, the plants were harvested and treated as described by Li et al. (2022) [46]. The dry weights (biomass) of roots and shoots were measured using an electronic balance. The plant samples were dried, ground, and digested with nitric acid and perchloric acid. Furthermore, the digestion solutions were reduced with hydrochloric acid, and the Se concentration was determined using a hydride generation-atomic fluorescence spectrometry (AFS-9700, Beijing Haiguang Instrument Co., Ltd., Beijing, China) [46]. The translocation factor (TF, Se content in shoots/Se content in roots) was calculated as described previously [47].

Statistical Analysis
The data were analyzed using SPSS 27.0 software (IBM, Chicago, IL, USA). Data were normalized and subjected to a homogeneity test. One-way analysis of variance was then conducted for comparison of the multiple groups, followed by Duncan's multiple range test for pairwise comparison (p < 0.05). Relationships of the biomass and Se content between the monoculture and intercropping treatments were evaluated using regression analysis. Pearson's correlation analysis was conducted to determine the correlations among the different indicators of each plant. Grey relational analysis was performed to explore the relationships of the different indicators with the shoot Se content of each plant as described by Wang (2019) [48] and Zhang et al. (2023) [49].

Conclusions
Intercropping of C. betacea with sect. Solanum inhibited the growth of C. betacea and promoted the growth of sect. Solanum as indicated by decrease or increase in their biomasses and photosynthetic pigment contents. In addition, intercropping C. betacea with sect. Solanum improved their resistances to Se by increasing the levels of antioxidant enzyme activities and soluble protein contents in the two plants. Moreover, intercropping of the two plants increased the shoot Se contents in C. betacea and sect. Solanum. The biomass and Se content of intercropped sect. Solanum exhibited linear relationships with that of monoculture, and the biomass and Se content of intercropped C. betacea showed linear relationships with that of sect. Solanum monoculture. CAT activity, POD activity, and soluble protein content were the top three indicators highly associated with the shoot Se content in C. betacea. The POD activity, soluble protein content, and TF were the top three indicators significantly associated with the shoot Se contents in sect. Solanum. Further studies should be conducted under field conditions to verify these findings, and the effects of Se uptake in C. betacea fruits should be evaluated.